Optical amplifiers amplify light as it propagates within a waveguide, such as an optical fiber. Optical amplifiers remove the need to convert the optical signal to an electrical signal with a photodetector, amplify the electronic signal, and then use the amplified electronic signal to modulate a laser to provide regeneration of the original optical signal. Optical amplifiers are used as repeaters, optical preamplifiers, and gain blocks within telecommunication systems in which optical fibers form the system backbone. Several types of optical amplifiers have been researched for use in telecommunication systems. Raman amplifiers require large optical pump power. Brillioun amplifiers are limited to communication systems at bit rates of less than 50 MHz. Semiconductor laser amplifiers have inherently lower signal to noise ratio and are often limited in gain.
Rare earth doped amplifiers are formed in waveguides including optical fibers. When the core of a waveguide is lightly doped (0.01 to 0.3 wt %) with rare earth ions, such as erbium, as is typical, the waveguide is operable to amplify a signal, although the waveguide must be long to provide the desired gain. Erbium doped fiber amplifiers require lengthy fibers wrapped around fiber spools, so they are large devices and unsuitable for integration in compact systems. For long haul telecommunication systems, the long erbium doped fiber lengths and the required banks of large optical pump lasers are acceptable. As the optical systems get closer to the homes and businesses, it is desirable for the amplifiers to be smaller, less expensive and integrated into the system.
The amplification provided by a rare earth doped amplifier increases with the doping level of the rare earth dopant in the waveguide. Silica glasses can absorb a limited concentration (up to about 0.01 wt %) of a rare earth dopant before the dopant precipitates out of the glass. When silica glass boules with high levels of rare earth dopants are drawn into fibers, the resulting fiber is very fragile and breaks more easily than an undoped silica fiber. Thus, the length of silica fibers that can be reliably obtained is limited when the silica is heavily doped with rare earth ions.
Moreover, as the rare earth dopant level increases in silica glass, the rare earth ions form ion clusters. When the rare earth ions are excited by an optical pump, part of the pump energy is exchanged between the ions in the cluster. This energy not used for amplification and the amplification process is quenched when such clusters form in the silica glass. It would be desirable to have a compact optical amplifier that would overcome the above disadvantages.